I always find it ironic that software that we are adviced to install as a gate to prevent malware and viruses from clogging up our systems, is itself a system clogger.
Terrorism or viruses, it all basically leads to the same scenario: creating a paranoid system that becomes so rigid and clogged with precautionary measures that its characteristics [to the end user/citizen] is not unlike the very thing we're trying to stop. In either case, one dedicated person is all it takes. [Ninja-EDIT: Sorry, just a somewhat off analogy, not meant as any kind of political statement at all.]
I know some universities advice Mac users to install anti-virus applications to prevent Windows viruses from being forwarded to Windows computers via e-mail, though if you receive a mail that doesn't mention that tasty looking - wait, I meant suspicious - attachment, do not open it and do not automatically forward the mail.
Frankly, the best anti-virus software is in your head. Use caution and sense ("common" ... what?). On a hilarous note, even those pirating software keep an eye out nowadays and screams from the top of their lungs in some comment thread that their pirated, "free" software is virus infested. Still, it's an ironic feedback system.